rear main seal replacement 73 Dart 318 V8

Done it many times, remove rear main and loosen the rest so the crank can drop down a little.
Tap the old seal in with a small punch until you can grab it on the other side and pull it out.
You can usually push the new one in by hand all or almost all the way. Help it in with the punch if not. Lube it up with oil.
Be careful to keep the seal tight to the crank as you rotate it in or the block will cut a slice off the outside edge of the seal.